Data defects may exist in optical recording medium such as compact discs, video compact discs or digital video discs due to imperfect recording quality, inadequate preserving condition or incorrectly operating the discs. The poor conditions of discs themselves, for example suffering from scratches or contaminants, are also likely to result in undesired data defects. The data defects could be rendered in various aspects. For example, a slicer in an optical reading system is used for converting the read analog signals into digital signals according to a default central level signal. The default central level signal is a predetermined voltage value stored in the optical reading system. When the read analog signal is higher than the default central level signal, the sliced digital signal will be a high level signal. When the read analog signal is lower than the default central level signal, the sliced digital signal will be a low level signal. If there is any data defect existing in the optical disc, the read analog signal will be affected by the data defect, so that the sliced digital signal will be obtained with errors. If the error-sliced digital signal is operated by the following data processing circuit and then is processed by the continuing servo tracking operation, it causes errors of the data processing or a crash of the optical reading system. For preventing the data-reading procedure from being affected by data defects, it is preferred to detect these data defects existing in the discs and take adequate measures in advance.
Please refer to FIG. 1 which is a schematic diagram showing the light-receiving portions of an optical pickup head. The optical pickup head 1 includes a master part 11 having four receiving portions A˜D, a first side part 12 having two receiving portions E and G, and a second side part 13 having two receiving portions F and H. The laser light emitted from the optical pickup head 1 is reflected by the optical disc. The reflected laser signal carries certain data stored in the focused spot of the optical disc. Then, the reflected laser signal is received by the eight receiving portions A˜H to be processed into useful signals. One of the useful signals is a sub-beam addition signal (SBAD) signal. The SBAD signal is substantially the summation of the sub-beam signals generated by the light-receiving portions E, F, G and H in response to the received light intensities. Conventionally, the optical reading system monitors the SBAD signal to realize the data-storage quality of an optical disc. If the level of the SBAD signal is lower than a predetermined level, it is determined that data defects occur. In the meantime, the subsequent servo tracking operation of the optical reading system is suspended to prevent from incorrect data processing of the data stored in the optical disc.
An example that there are serious and small scratches existing in the data storage region of a disc is given with reference to FIG. 2(a) in which the relationships among SBAD, data and defect-indicating signals are shown. Due to a serious scratch in the data storage region of the disc, a data signal drop E11 of the data signal E1 is rendered, and a signal drop S11 occurs in the SBAD signal S1 correspondingly. Since the signal drop S11 has been down lower than a preset threshold level L1, the defect-indicating signal D1 is switched to a high level D11 to suspend the servo tracking operation of the optical reading system, thereby protecting the optical reading system from errors or hanging. After the scratch has been passed and the defect-indicating signal D1 is switched back to a low level, the servo tracking operation of the optical reading system is restored. The preset threshold level L1 can be a voltage value stored in the optical reading system.
Afterwards, another scratch that is less serious in the data storage region of the disc is encountered, so a less data signal drop E12 of the data signal E1 is rendered. Correspondingly, a less signal drop S12 occurs in the SBAD signal S1. Since the signal drop S12 has not been down to the preset threshold level L1, the defect-indicating signal D1 will not be switched to a high level D11 to suspend the servo tracking operation of the optical reading system. Instead, the defect-indicating signal D1 stays low as indicated by the reference D12.
Unfortunately, in a case that the SBAD signal S1 cannot reflect the real defect situation, e.g. data interruption E21 and E22 occurring in the data signal E2 as exemplified in FIG. 2(b), errors may happen. Since the signal drops S21 and S22 of the SBAD signal S2 corresponding to the data interruption E21 and E22 are insignificant, i.e. not lower than the preset threshold level L1, there will be no high-level peaks occurring in the defect-indicating signal D2, neither in the corresponding section D21 nor in the corresponding section D22. Accordingly, the subsequent data processing circuit and the servo tracking operation keeps on processing the sliced digital data. Therefore, the signal drops S21 and S22 of the SBAD signal S2 corresponding to the data interruption E21 and E22 will affect the optical reading system and causes the incorrect data processing of the data stored in the optical disc or a crash of the optical reading system.